1. Technical Field of the Invention
The present invention relates to a polishing machine for polishing a periphery of a sheet when a rotating grinding stone is made to come into contact with a periphery of a rotating sheet. More particularly, the present invention relates to a polishing machine for polishing a periphery of a sheet suitably used for polishing a periphery of a sheet such as a semiconductor wafer, in the periphery of which a notch or orientation-flat marking is formed.
2. Background Art
A surface of a semiconductor wafer, which has been cut off by means of slicing, is polished, and a periphery of the semiconductor wafer is also polished in order to prevent the occurrence of cracks and the adhesion of dust. In this case, the periphery of the semiconductor wafer is polished in such a manner that an oblique face of a rotating grinding stone having grooves is pressed against the rotating semiconductor wafer so that a rotary shaft of the semiconductor wafer and that of the grinding stone can be parallel with each other.
However, in the above conventional method of polishing the periphery of the semiconductor wafer, as the abrasive grains move only in the circumferential direction of the semiconductor wafer, stripes are formed on the polished face in the periphery of the semiconductor wafer by partial blades of the grinding stone, that is, it is impossible to provide a polished face, the surface roughness accuracy of which is sufficiently high. When the surface roughness accuracy of the polished face of the periphery is insufficient as described above, the following problems may be encountered. A surface of the periphery is locally cracked and chips are generated from the cracks. Accordingly, dust adheres onto the surface of the periphery. Further, fine powder gets into the cracks, which could be a cause of generating dust. Furthermore, cleaning water staying in the cracks is vaporized in a later process, which has a bad effect on the after-process of manufacturing the wafer. In order to solve the above problems, the following countermeasures are taken. The grain size of the grinding stone is reduced to be more minute, the quantity of cutting is decreased, the number of times of dressing is increased, or the surface roughness accuracy of the polished face is enhanced by changing over several grinding stones (Two stages of grinding stones or three stages of grinding stones are used.). However, the effects provided by the above countermeasures are limited. Further, when the above countermeasures are adopted, the grinding efficiency is deteriorated.
Therefore, the present applicant has proposed the following technique which is disclosed in Japanese Examined Patent Publication No. 2876572. While a rotary shaft of a grinding stone is being tilted in the tangential direction of an outer circumference of a semiconductor wafer, a peripheral edge of the semiconductor wafer is polished. Due to the foregoing, a direction of the motion of abrasive grains of the grinding stone is tilted with respect to a polished face of the semiconductor wafer, and the occurrence of stripes on the polished face caused by partial blades of the grinding stone can be prevented. As a result, polishing can be executed with high accuracy.
However, the above polishing method is disadvantageous as follows. According to the above polishing method, the rotary shaft of the grinding stone can be tilted only in one direction. Therefore, when the semiconductor has a notch portion or orientation-flat marking portion, it is impossible to successfully polish the peripheral edge of the notch portion or orientation-flat marking portion.